1. Field of the Invention
This invention relates to electrical surge arresters, or diverters, and more particularly but not solely to electrical surge arresters for use in electrical power generation, transmission and distribution systems to protect such systems against power surges caused, for example, by lightning, and against over-voltages caused, for example, by switching operations.
2. State of the Art
Electrical surge arresters or diverters are well known for protecting equipment such as electrical power distribution systems and are generally connected in parallel with the equipment to be protected. A typical surge arrester provides a high or infinite impedance during normal system voltages in order to minimize steady-state losses. During surges, the arrester provides a low impedance in order to limit the voltage, and dissipates or stores the energy in the surge without damage to itself. After the passage of the surge, the arrester returns to open-circuit conditions.
A widely-used surge arrester comprises a plurality of non-linear voltage-dependent resistors contained within the bore of an externally shedded glazed porcelain insulator housing. The resistors are generally separated by discharging or spark gaps. During normal operating conditions the arrester has an infinitely high resistance so as to minimize steady-state losses of the equipment. However, in the event of a surge, the resistance of the arrester is substantially reduced such that the voltage is limited to acceptable levels to prevent damage to associated equipment, whilst the power follow current is sufficiently restricted to a level that can be cleared by the spark gaps.
The surge arrester described above is generally effective. However, under certain circumstances, the porcelain insulator housing may shatter, thereby scattering high temperature fragments, which is clearly dangerous.
Another type of electrical surge arrester, developed in order to overcome the problems associated with the arrester described above, consists of a unitary structural core comprising alternately stacked metal oxide varistor blocks and aluminium alloy heat-sink/spacer blocks. The opposed electrode surfaces of the individual varistor blocks are formed with metallised aluminium contacts and their sides are coated with an insulating material. The electrode surfaces of respective blocks are held in face-to-face physical and electrical contact by means of a silver loaded epoxy. The stack of blocks is coated with a glass-reinforced plastics shell and the whole assembly is encased in a heat-shrink or polymeric sleeve formed with alternating sections of greater and lesser diameter to provide `sheds` for `creepage`. In order to ensure that the interface between the heat-shrink sleeve and the glass-reinforced shell around the core is void-free, a mastic sealant is used within the heat-shrink sleeve. Finally, stainless steel end caps are provided at either end of the core as terminations. The surge arrester thus described operates in a similar manner to the type having a porcelain insulator housing, but has the added advantage that it has a non-explosive failure mode. It is relatively light, but is strong, resistant to damage and is unaffected by atmospheric pollutants or moisture ingress.
However, the latter surge arrester is of relatively complex construction and is expensive to manufacture. Another disadvantage of such a surge arrester is that, because the amount of energy dissipated by the device is dependent upon the size and number of varistor blocks, the device is often relatively large in order to accommodate particular applications. Further, air or moisture may become trapped between the glass-reinforced shell and the polymeric sleeve during manufacture, which may result in undesirable ionization effects.